Alternatives to systemd

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This page describes the various init systems which are available as alternatives to systemd
For a more comprehensive explanation of init, and runlevels, see wikipedia.org Page: Init

Init is the first process started during system boot. It is a daemon process that continues running until the system is shut down. Init is the direct or indirect ancestor of all other processes, and automatically adopts all orphaned processes. It is started by the kernel using a hard-coded filename; if the kernel is unable to start it, panic will result. Init is typically assigned process identifier 1.

The init scripts (aka rc runtime configuration scripts) are launched by the init process to guarantee basic functionality on system start and shutdown. This includes (un)mounting of file systems and launching of daemons. A service manager takes this one step further by providing active control over launched processes, or process supervision. An example is to monitor for crashes and restart processes accordingly.

These components combine to the init system. Some init systems incorporate the service manager in the init process or have init scripts in close relation to them. Below, such init systems are referred to as integrated, although entries in different categories may explicitly depend on each other.

A nice (but still non-comprehensive) overview of init systems can be found in this blog entry, titled "A history of modern init systems (1992-2015)"

Contents

init systems

System Description Latest release
BusyBox init quite small init without runlevels, solely signal driven IPC (no fifos, sockets, SysV IPC) 2018-07-31
dinit dependency based C++ init with process supervision, roll-back, and socket activation 2018-07-12
epoch sequential, non-parallel init without dependency tracking designed for minimal footprint and unified configuration 2015-06-23
finit fast, event based, modular, extensible (C hooks/plugins) init with SysV runlevels, proper daemon supervision and logging, and optional builtin getty and inetd 2018-01-23
initng (2) modular, extensible, parallel, asynchronous, dependency based init 2007-03-25
myinit parallel init with dependency tracking via reference counting (akin to the "need" concept) lacking proper supervision (respawn only, no output logging) 2011-07-11
nosh exec chaining, dependency based daemontools family C++ init and supervision suite with reliable logging, (console) virtual terminal management, systemd unit file compatibility; can also be used as service manager under a different init 2017-12-11
pies dependency based GNU init daemon and super server with SysV runlevels, daemon supervision and logging, inetd functionality, socket activation, fine grained per service access control, understands SysV inittab, inetd.conf, and MeTA1 config files 2016-10-01
procd (2) OpenWrt init and process/service management daemon with ubus integration 2018-07-30
sinit Simple init initially based on Rich Felker’s minimal init 2015-06-16
sninit Small init implementation with SysV init like (sub)runlevels 2015-12-31
SysV init (2) Traditional System V init. New release: v2.91 on July 07 2018 (git commit log) 2018-07-07
ToyBox init (2) similar to/(almost) compatible with BusyBox init (same config syntax/file) 2018-06-24
ToyBox oneit (2) very simple init launcher (just (re)spawns a single child process and reacts to incoming signals) 2018-06-24
ueld simple configuration, solely signal driven (akin to BSD/BusyBox/ToyBox init) 2017-06-24
uinit Smallest init possible 2017-05-16

abandoned/defunct/discontinued/dormant/halted init projects

Source code is available for download to anyone interested (in reviving them). Some of the projects listed below still compile/work while others may need some tweaking to make them work (again).

System Description Latest release
cinit (2, 3) dependency based parallel init without subprocess output logging ala minit/daemontools 2009-09-29
daemond daemon control and init daemon written in C++ (akin to dinit) 2003-11-14
depinit supports parallel execution, dependencies, true roll-back, log pipelines, improved signaling, and unmounting of filesystems on shutdown 2003-10-06
einit (2, 3, 4) modular, parallel, asynchronous, dependency based init system; XML (service) config files 2007-12-18
ettcl custom Tcl-7.6 based interpreter to run the ETLinux init, crond, httpd, and telnetd daemons written in Tcl 2002-??-??
jinit C++ init daemon supporting dependency tracking akin to the "need" concept 2003-04-22
minit small dependency ordered, parallel init with proper subprocess supervision and logging 2005-??-??
ninit dependency based parallel init with supervision, logging, and time-based scheduling akin to crond (HOWTO) 2010-01-16
pinit (sf) XML configuration, dependency based fast parallel boot, modular design; uses Glib 2.0 and LibXML 2 2003-05-01
serel (sf) parallel service launcher with synchronisation and integrity-checking (primarily to speed up SysV init boots, akin to startpar) 2002-08-06
simpleinit(-msb) dependency based (implements the "need" concept) sequential init without proper supervision (respawn only, no output logging); shipped with util-linux until v2.20; the msb fork is still used in Source Mage Linux 2007-11-08
System XVI modular, self-healing, and interface-oriented service manager and init system 2016-05-18
twsinit tiny init largely written in x86 assembler 2003-09-19
upstart "event based" init that reacts eagerly to incoming "events" to emulate dependency tracking; leaks memory 2014-09-04
uselessd (2) forked from systemd v208 2015-01-06

process supervisors (overview) and service managers

service manager: a suite of programs which start and stop services, both long-running daemons and one-time initialization scripts, in the proper order according to a dependency tree

Program Description Latest release
anopa service management suite built around s6; can be used as init via exec chaining 2017-06-20
Circus (doc) runs, controls and monitors processes and sockets; written in Python; uses ZeroMQ 2018-06-15
Comar Python configuration/network/service manager with dbus integration used in Pardus Linux 2011-10-24
daemontools collection of tools for managing UNIX services 2001-07-12
daemontools encore an enhanced version of daemontools (github repository) 2014-04-02
freedt an independent daemontools reimplementation 2014-09-03
god (2) an easily configurable, extensible, monitoring framework written in Ruby 2014-03-06
MiniBase small userspace utilities for Linux; includes a process supervisor that can be used as stage 2 of process #1 via exec chaining akin to s6-svscan 2018-02-10
Monit process and system monitoring daemon; system status is viewable directly from commandline, or via a native HTTP(S) web server 2018-05-29
Mudur Python init/rc script used in Pardus Linux 2011-10-06
OpenRC (2, 3) dependency based service management suite; includes a process supervisor and a simple init alternative 2018-08-06
perp persistent process (service) supervisor and managment framework for UNIX 2013-01-11
procer process supervisor developed for mongrel2 2014-03-18
restartd Debian process-restarting daemon 2013-01-11
runit process supervision suite in the line of daemontools; includes a simple init (2) replacement 2014-08-10
s6 small process supervision suite for UNIX ala daemontools, runit, and perp; can be used as stage 2 of process #1 via exec chaining 2018-08-14
s6-rc service management suite for s6 2018-08-20
Shepherd GNU service manager and init daemon written in Guile (formerly known as GNU dmd) 2018-09-26
ssm simple service manager bash scripts for Linux 2016-03-31
Supervision Framework OpenRC-like service management suite for daemontools(-encore), runit, and s6 2018-09-01
supervisord (doc) process control daemon written in Python 2018-02-15
systemctl-replacement drop-in reading systemd service descriptions, with docker-oriented init function, written in Python 2018-09-10

SysV init enhancement addons

System Description Latest release
insserv can be used with SysV -based init systems; provides dependency-driven system startup (dependencies are specified by LSB headers within init.d scripts) 2012-11-14
startpar can be used by the SysV RC boot system executor to allow parallel process system startup 2014-02-09

Safer, supervision-friendly replacements for crond and atd

System Description Latest release
bcron seperate tools for different tasks with strictly controlled communication between them; crontab compatible 2015-08-12
slicd (github) modular design (seperate tools); crontab compatible 2016-10-30
uschedule job scheduling suite that uses seperate tools for different tasks 2004-08-??

udev alternatives for automatic Linux device file creation and management

udev (userspace /dev) is a device manager for the Linux kernel.

In April 2012, udev's source code was merged into the systemd source tree.[1]

Although udev can still be compiled for usage without systemd, Lennart Poettering said that they will not polish udevd outside of systemd[2], adding:

"Yes, udev on non-systemd systems is in our eyes a dead end, in case you haven't noticed it yet. I am looking forward to the day when we can drop that support entirely."

So to be on the safe side, you probably want to use an alternative to udev:

DevMan Description Last release
eudev (2, 3) Gentoo's maintained fork of udev 2017-11-22
mdev the plug-and-play manager built into BusyBox 2018-07-31
mdevd (github) nonforking Linux kernel uevent handler daemon; uses the mdev config file format 2018-08-14
nldev (2) Suckless netlink frontend for mdev; replacements for udevd and udevadm (see also nlmon) 2018-08-03
smdev mostly mdev-compatible Suckless program to manage device nodes 2015-04-12
vdev (github) portable UNIX userspace device-file manager 2016-08-08

For more see this StackExchange question.

Related helper tool: udevil (2) (un)mounts removable devices without a password, shows device info, and monitors device changes. It can also mount ISO files, nfs://, smb://, ftp:, ssh:// and WebDAV URLs, and tmpfs/ramfs filesystems. Uses Glib and libudev (i. e. requires (e)udev) and can work completely without systemd, consolekit, policykit, dbus, udisks, gvfs & fuse (although it can coexist with any of these).

See also

Init-specific articles:

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